The present invention relates to highly radioactive, miniaturized, cylindrical strontium 90 titanate, strontium 90 zirconate, and strontium 90 silicate radiation sources having an activity exceeding 25 mCi/mm3, preferably xe2x89xa730 mCi/mm3, and a diameter less than 0.7 mm, preferably less than 0.4 mm. Another subject of this invention is a method for the production of these extremely small, but highly radioactive radiation sources.
In the following description, strontium 90 titanate is also referred to as 90SrTiO3, strontium 90 zirconate as 90SrZrO3, and strontium 90 silicate as 90SrSiO3.
With respect to medical applications, the importance of miniaturization of the radioactive radiation sources is steadily increasing. For example, in tumour therapy and intravascular brachytherapy, i.e. the exposure of the inner wall of blood vessels to radiation, inserted miniaturized sources are used.
There are essentially two known methods of producing miniaturized radiation sources of the strontium 90 isotope. In the production of tabular radiation sources, a mixed precipitation of Ag2CO3/90SrCo3/TiO2 with subsequent malleabilization of the precipitate is used wherein the emerging silver cake is brought into the desired shape. Regarding the production of miniaturized, cylindrically shaped strontium 90 sources, it is known to soak a pre-formed carrier body consisting of titanium dioxide with a 90Sr(NO3)2 solution, to dry and then to anneal it at a temperature exceeding 1,000xc2x0 C. In this process, insoluble strontium 90 titanate (90SrTiO3) is generated. These radiation sources are characterized by having an activity of only 5 to 7 mCi per mm3. This activity and the resulting dose rate, however, are not sufficient, for instance, for the aforementioned medical applications. There still remains a need of having possibly small but highly radioactive strontium 90 radiation sources.
Therefore, it was an object of this invention to provide a manufacturing method for producing highly radioactive and very small strontium 90 radiation sources using a possibly automated or partly automated technique. In order to reach even very small blood vessels in medical applications, the diameter of the radiation source should be less than 0.6 mm.
The object of this invention is solved by a method for producing ceramic strontium 90 radiation sources wherein the aqueous solution of a strontium 90 salt is united with a titanium, zirconium, and/or silicon compound being in a dissolved state and the solution of one or more ammonium salts of carbonic acid and/or a low-molecular organic acid, the solvent is expelled from the mixture, the residue is calcinated and, after adding auxiliary agents, is transformed into a plastic state, the plastic matter is microextruded, the emerging thread is exposed to a sintering process and finally cut at the desired lengths so that miniaturized radiation sources are obtained which can be encapsulated in a manner known per se if necessary. It is also possible, of course, to first cut and then sinter the strontium 90 mass thread obtained. Below, the 90SrTiO3, 90SrSiO3, and 90SrZrO3 bodies produced according to the present invention are also referred to as xe2x80x9cradioactive ceramicsxe2x80x9d.
The manufacturing method according to the present invention by which the radioactive ceramics is produced by microextrusion is advantageous in comparison with the conventional soaking technique in which pre-fabricated inactive ceramic carriers are soaked with the strontium 90 solution in that radiation sources having a higher Sr 90 portion (in the case of 90SrTiO3, the density is xe2x89xa74 g/cm3) can be produced. The method according to the present invention may (in part) be automated and remote-controlled. No grinding processes, no screening, no filtration processes, no spraying operations, and, except cutting, no finishing processes are necessary. The cylindrical sources are not manufactured as individual cylinders but as string (thread) which is cut in the raw or sintered state.
The initial compounds for the manufacturing method according to the invention are commercially available. For instance, strontium 90 nitrate having a concentration of 0.2 g solid matter/ml, which is commercially available and contains portions of barium nitrate and minor iron impurities, can be used as strontium 90 salt. The strontium 90 salt used may also be the salt of a low-molecular organic acid such as for instance 90Sr formiate or 90Sr acetate.
Although the present invention allows the use of water-soluble salts such as chlorides as titanium, zirconium, or silicon compounds, alcoholates are preferred. Mixtures of titanium, zirconium, and silicon alcoholates may also be used here so that mixed ceramics are generated, for instance comprising 90SrSiO3 and 90SrTiO3, or 90SrSiO3 and 90SrZrO3. The embodiment using either a titanium or a zirconium or a silicon alcoholate is preferred, however. According to the present invention, ethylates, propylates, butylates, the corresponding iso-compounds, or the corresponding mixed alcoholates are used as preferred alcoholates. Special preference is given to tetra-isopropyl-orthotitanate (TiPOT) in the production of 90SrTiO3 ceramics. For producing 90SrSiO3 ceramics, tetraethoxysilane (TEOS) is particularly preferred. For producing 90SrZrO3 ceramics, zirconium (IV) propylate is particularly preferred. The alcoholates are, according to the present invention, preferably used in an anhydrous alcoholic solution.
As an ammonium salt, all those compounds may be used the anion of which is thermally separable or thermally decomposable and which form a hardly soluble compound with strontium, such as carbonate or oxalate. The ammonium may also be present in a substituted form as an organic ammonium compound. Ammonium compounds soluble in alcohol such as ammonium oxalate which may be used in a solution together with the silicon, titanium, and zirconium alcoholates are also suitable. In a preferred embodiment, (NH4)2CO3 is used.
According to the present invention, the mol ratio of 90Sr:Me:NH4 is 0.85-1:0.95-1.05:1.7-2, preferably 0.93:1:1.86, wherein Me means Ti, Zr, and/or Si.
The initial solutions described above are mixed by starting with the 90Sr solution and homogenized, preferably by stirring. Thereafter, the solvent is mostly expelled and the residue calcinated, preferably at 650-1,000xc2x0 C. wherein the duration period at this temperature is approximately one hour. The preferred calcination temperature ranges between 800-830xc2x0 C., in particular preferably at 820-830xc2x0 C.
The expulsion of the solvent may be accomplished by evaporation and/or sublimation.
Afterwards a plasticator is mixed into the calcinated mass. A number of recipes of plasticators for oxide ceramics are known which usually include organic auxiliary substances such as a solvent, a bonding agent, a softener, a lubricant, and a dispersion agent. One substance may also fulfil the function of several components.
An aqueous plasticator comprising a cellulosic derivative of a medium mol mass, a polysaccharide, a polyol, e.g. glycerol, and a polyelectrolyte has proved to be advantageous for the plastication of the strontium 90 mass according to the present invention. These auxiliary agents are added to the calcinated powder after cooling in a quantity ranging between 6 and 18 percent by weight in relation to the weight of the powder. Apart from these auxiliary agents being per se usual with respect to plastication, according to the present invention a silicon, titanium, and/or zirconium alcoholate in a quantity between 0.5 to 2 percent by weight is added to the calcinated powder during the plastication process. The alcoholates used may be the same as mentioned above in connection with the production of the initial mixture. In case of the use of TiPOT, the mass ratio of cellulosic derivative:polysaccharide:polyol:polyelectrolyte:TiPOT is 7-9:3.5-4.5:6-8:0.8-1.2:15-24, preferably 8:4:7:1:19. In case of the use of TEOS, the mass ratio of cellulosic derivative:polysaccharide:polyol:polyelectrolyte:TEOS is 7-9:3.5-4.5:6-8:0.8-1.2:20-30, preferably 8:4:7:1:25.
The crumbly mass mixed with the plasticator is then made smooth and pore-free by intense kneading and deaerating and in a final step is microextruded. For microextrusion devices may be used which operate in compliance with the principles of common capillary viscosimeters or common laboratory extruders.
The subsequent sintering of the strontium 90 ceramic string is preferably accomplished by slowly heating up to approximately 400xc2x0 C. and then speeding up the heating process a little until the proper sintering temperature is reached which ranges between 1,260xc2x0 C. and 1,420xc2x0 C. In a most preferred embodiment heating up to approximately 400xc2x0 C. is done by 1.5 K/min and then up to the sintering temperature by about 5 K/min. It has been found out that the preferred sintering temperature ranges between 1,370 and 1,390xc2x0 C. The sintering process shall proceed for about one hour. Thereafter, the strontium 90 thread is cut to the desired lengths, for instance by laser cutting. The length of the radiation sources is preferably approximately 1.8 mm. Other lengths may of course also be realized.
The 90SrTiO3, 90SrZrO3, or 90SrSiO3 radiation sources obtained are of sufficient stability and have densities xe2x89xa780 % of the crystallographic density which corresponds to a radioactivity  greater than 25 mCi/mm3, preferably even xe2x89xa730 mCi/mm3. The diameter obtained is less than 0.6 mm, preferably also less than 0.4 mm. The particularly preferred diameter of the sources produced according to the present invention is approximately 0.3 mm. Statistically, the strontium distribution lies within molecular ranges. The final products of the method according to the present invention are abrasion-proof; the strontium 90 is not washed out by water or other solvents. The final products are highly homogenous. If it is desired to further enhance homogenity, this can be accomplished by lyophilizing the initial mass after the expulsion of the solvent in an additional interim step and then calcinating the lyophilisate. The other steps of the method are performed as described above.
Beside the production method described above, radioactive strontium titanate, strontium zirconate, and strontium silicate radiation sources having an activity exceeding 25 mCi/mm3, preferably xe2x89xa730 mCi/mm3, and a diameter  less than 0.7 mm, preferably  less than 0.4 mm, most preferably  less than 0.3 mm, constitute another object of the present invention. The cylindrical radiation sources according to the present invention may be encapsulated in a per se known manner in a material tolerated by the human body such as for instance stainless steel. This is accomplished by inserting the radioactive ceramics produced into a small tube which is closed on one end and sealing the opening on the other end by means of a lid. Preferably, said lid is laser-welded.
In order to improve the visibility of the radiation sources according to the present invention during the therapy in X-ray diagnostics, two tantalum cylinders having the same thickness as the ceramics may be inserted as X-ray markers at both ends, respectively, of the cylindrical radioactive ceramics into the small tube which is closed on one end. Then the tube is sealed by means of the lid as described above. In this manner, it is possible to show/determine the orientation of the radiation source because stainless steel and ceramics are invisible in X-ray diagnostics. Due to the extreme tininess of the radiation sources produced, it is not possiblexe2x80x94as for instance in seeds for prostata cancer irradiationxe2x80x94to insert silver or gold threads as X-ray markers. With respect to the present case, the method using tantalum cylinders as described above hence provides an excellent solution.